In a modern display system data from multiple image sources are displayed. These images are shown on the display using a "window", a technique in which an area of a display is assigned to an image source. However, problems arise when the outputs from multiple sources must be coordinated on the display. When there is more than one source for the display, a mechanism is required to coordinate the output to a single display monitor.
A straightforward solution is to design the system such that only one source can be displayed at a time. For example, if a High Definition Televisions (HDTV) sampler input is selected, then only the HDTV output is output to the display. However, with an increased demand for "windowing" in graphics systems, and with an increased emphasis on a multimedia environment, where there exists more than one video source, this relatively simple solution is not adequate.
In order to accomplish windowing there must be provided a mechanism such that, within the total area defined by a monitor screen, different sub-areas are sourced by different video sources. FIG. 1 shows an example of a windowed graphics monitor screen. Area A may be sourced by a local host or workstation. Area B may be sourced by a remote graphics server connected via a high speed network. Area C may be sourced by a HDTV sampler. One technique to provide such a display is known in the art as pixel switching. That is, for a given display area, or window, the source of pixels for that area is selected from a specified image plane.
A more complex problem is presented when an arbitrarily shaped image from a source is overlayed on top of another image, rather than over a simple rectangular window. For example, an image of an automobile may be rendered using a graphics server, and the automobile image may be required to be overlayed upon a HDTV-generated background image. This requires that all pixel selection be accomplished on a pixel-by-pixel basis, since the shape of the foreground object, or automobile, is not rectangular.
One solution that is applicable to a two-image source system utilizes color keyed pixel switching, and allows pixel-by-pixel selection between two sources. This technique is described in commonly assigned U.S. Pat. No. 4,994,912, entitled "Audio Visual Interactive Display", by L. Lumelsky et. al.
However, for the case where there are more than two video sources, such as is illustrated in FIG. 1, a different solution is required for displaying N, where (N&gt;2), image sources on a pixel-by-pixel basis.
Another problem that arises in overlaying arbitrarily shaped multiple source images is due to an aliasing effect resulting from image pixel switching. In that a pixel of one image source may not blend with a pixel from a second image source, aliasing results. Aliasing causes the resultant image to exhibit undesirable artifacts along the boundary between the foreground and the background images, such as stair-casing and color distortion. In order to eliminate the artifacts, an anti-aliasing technique is necessary. However, in that the foreground image does not contain information about the background image, the anti-aliasing should be accomplished in real-time at the video output. Thus, a technique for anti-aliasing N image sources in real-time is required. Furthermore, in order to anti-alias an arbitrarily shaped foreground object, the anti-aliasing must be accomplished on a pixel-by-pixel basis.
However, in modern high resolution displays the video data bandwidth and data rates are very high, thus placing severe timing constraints on any pixel processing that is to be accomplished in a real-time manner.
In U.S. Pat. No. 5,001,469, issued Mar. 19, 1991, entitled "Window-Dependent Buffer Selection" to Pappas et al. there is described window control hardware in a graphics sub-system in which multiple image sources are shown on to a single monitor. This is accomplished by defining each window as a separate frame buffer and defining for each window, i.e. frame buffer, a window identification, and a window size and location based on four values of top location, bottom location, left location, and right location. This system also employs a prioritizing scheme where N frame buffers ("windows") are prioritized from 0 to N-1, where 0 has the highest priority and N-1 has the lowest priority. The graphics sub-system includes N window detection logics, one for each frame buffer, which use comparators for window size and location values to determine if the associated window is active for a region of the screen. If the window is active an image source pointer and other information are sent to prioritizing logic which prioritizes N input signals to determine which "active" image source has a highest priority. An active image with the highest priority is chosen by the priority logic and shown on a monitor.
Pappas et. al. employ window size and location values to control multiple image sources, and an image frame buffer does not contain multiple windows. Furthermore, this system appears to be limited for use with only rectangularly shaped windows. Also, the problem of displaying multiple sources with differing image formats is not addressed.
It is thus one object of the invention to provide for the simultaneous display of video data from N independent image sources, where N may be greater than two, through the use of pixel switching and control on a pixel-by-pixel basis for the N image sources.
It is another object of the invention to provide a method and apparatus for displaying video data from a plurality of image sources on a monitor using a combination of alpha mixing and pixel switching, on a pixel-by-pixel basis, based on pixel color keying and window identification.